For my own needs and my own experiences with heavy traffic sites (200k+ pageviews per day), I find SSI to be very convenient. There aren't many easier ways to coordinate the production of a complex site by numerous people. Also in some circumstances, SSI can be faster than dynamic pages. If the creation of each page involves some nasty database queries or calculations, SSI is probably a faster, better solution.

However, if you are the only person making this site tick (or one of only a few people), then dynamically served pages may be your best bet -- especially if each web user will log in and see different information/prices. Depending on the number of different user types you have, the large number of SSI files could become a real pain to maintain. You can always use some simple HTML templating to keep the *look* of your site easily maintainable.

Just keep your slowest scripts' runtimes under 2 seconds. Granted 2 seconds is a *long* time for any dynamic page script to run, but this time is relative to your environment. My general rule of thumb: >1 sec, you need to optimize some code. >2 seconds, you're probably doing something nasty in the code. And if you have the RAM, get really aggressive with caching on your database. That should help keep the runtimes down.

You'll always have security issues to deal with. So, just be careful.